Write up a brief synopsis of your comic book plot. Comic books traditionally number 32 or 64 pages, each encapsulating either an entire story or a single chapter in a larger arc. Your comic can be longer or shorter if you wish, but it should still be reasonably complete--ending either with a proper sense of closure, or with a cliffhanger that segues easily into the next issue. Your synopsis should contain a brief explanation of the major events you want your comic book to encapsulate.
Break the plot down by page, covering what needs to be shown on each page in order for the story to flow properly. The average comicbook page contains from 4 to 8 panels to cover all the action, although you can go with more or fewer panels if it is dramatically appropriate. The panels need to convey every action or piece of dialogue required for the page; knowing what needs to be covered ensures that you don't pack one page with too much or too little information.
Write the script frame by frame. Describe each frame on each page, covering the required visuals, dialogue, narration and sound effects. As with Step 2, you need to be careful not to overload any frames. Make sure the dialogue is sparse enough to fit in a given frame without covering any of the important visual elements (such as the speaker's face). You may also wish to break the dialogue down by individual word balloons to confirm it can all fit.
Revise and polish your script to smooth out any rough spots. Correct the spelling, eliminate excessive frames and make sure the action moves logically from one frame to the next so that the reader can follow the story easily. When you're done, you can turn to script over to the artist for penciling.